UNDERSTANDING UPS OPERATION
Eaton 9395 Plus 1 UPS (225–275 kVA) Installation and Operation Manual
S
164201710 Rev 2
www.eaton.com\powerquality
6-5
Static
Switch
K5
Rectifier
Inverter
K1
K3
Battery
Converter
Battery
Battery
Breaker
Main Power Flow
Trickle Current
Energized
De-Energized
Closed
Open
Breakers
Contactors
K2
Bypass Input
Rectifier
Input
Output
Input
Breaker
(CB1
Figure 6‐3. Path of Current Through the UPS in Bypass Mode
In Bypass mode, the output of the system is provided with three-phase AC power
directly from the system input. While in this mode, the output of the system is not
protected from voltage or frequency fluctuations or power outages from the source.
Some power line filtering and spike protection is provided to the load but no active
power conditioning or battery support is available to the output of the system in the
Bypass mode of operation.
The internal bypass is comprised of a solid-state, silicon-controlled rectifier (SCR)
static switch (SSW) and a backfeed protection contactor K5. The static switch is rated
as a continuous-duty device that is used anytime the inverter is unable to support the
applied load. The static switch is wired in series with the backfeed protection
contactor, and together they are wired in parallel with the rectifier and inverter.
The static switch, being an electronically-controlled device, can be turned on
immediately to pick up the load from the inverter while the inverter output
contactor K3 opens to isolate the inverter. The backfeed protection contactor is
normally always closed, ready to support the static switch unless the bypass input
source becomes unavailable.
If the UPS transfers to Bypass mode from Online mode due to any reason other than
operator intervention, the UPS automatically attempts to transfer back to Online mode
(up to three times within a ten minute period). The fourth transfer locks the critical
load to the bypass source and requires operator intervention to transfer.